Resistant starch functions as a laxative and consuming it can lead to excessive wind. It is widely promoted as a weight loss aid, but the claims made for it are not supported by medical evidence. Resistant starch is being studied for possible health applications but most research is at an early stage; there is some evidence it might be a useful substitute for refined carbohydrates in diets aimed at reducing the risk of type 2 diabetes, and that it might help maintain a healthy colon.

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Resistant starch (RS) is any starch or starch digestion products that are not digested and absorbed in the stomach or small intestine and pass on to the large intestine.[2] RS has been categorized into four types:[2]

RS1 Physically inaccessible or undigestible resistant starch, such as that found in seeds or legumes and unprocessed whole grains.

RS2 Resistant starch is inaccessible to enzymes due to starch conformation, as in high amylose corn starch

RS3 Resistant starch that is formed when starch-containing foods are cooked and cooled, such as pasta. Occurs due to retrogradation, which refers to the collective processes of dissolved starch becoming less soluble after being heated and dissolved in water and then cooled.

Fermentation of resistant starch may produce gas and bloating when high quantities are consumed.[3] One review estimated that daily intake of resistant starch may be as high as 45 grams in adults,[4] an amount exceeding the total recommended intake for dietary fiber of 25-38 grams per day.[5] Resistant starch may contribute to colon health by producing short-chain fatty acids, among which butyrate is a primary energy source for colonic cells.[6]

In its various forms, resistant starch is digested and/or fermented variably,[7] leading to preliminary research of resistant starch subtypes on disease risk.[8] For example, although effects on weight management have been implicated, there is no evidence that resistant starch has an effect on human weight or energy balance.[9][10] Despite the lack of evidence, resistant starch has nevertheless been promoted as a "weight loss wonder food".[9]

There is preliminary evidence that resistant starch, used as a substitute for refined carbohydrate, may reduce the risk of type 2 diabetes.[11]

Resistant starch is considered both a dietary fiber and a functional fiber, depending on whether it is naturally in foods or added.[12][13][14] The U.S. Institute of Medicine has defined total fiber as equal to functional fiber plus dietary fiber,[15] and U.S. food labeling doesn't distinguish between them.[16]

In 1971, Painter and Burkitt suggested[19] that a significant gap exists between the amount of dietary fiber urbanized people consume and the optimal amount of fiber for health and wellness, but some skepticism remains.[20][21][medical citation needed] In 1982, Englyst et al.[22] gelatinized starch then post-processed it with both alpha-amylase and pullulanase in order to analyze it, found that some starch remained, and called it resistant starch.[23][24] In 1986, Berry formed[25] functional RS3[26] dietary fibers by a process of heating and cooling[27] a variety of starch sources, one of which was amylopectin pre-processed with the enzyme pullulanase. That source had the second highest alpha-amylase resistant starch level, while amylomaize or high-amylose starch had the highest.[28][29][30] In 2007, the Federal Register published a 2001 U.S. Institute of Medicine (IOM) Panel on the Definition of Dietary Fiber's response to a request from the U.S. Food and Drug Administration. The IOM Panel proposed two definitions: functional fiber as "isolated, nondigestible carbohydrates that have beneficial physiological effects in humans", and dietary fiber as "nondigestible carbohydrates and lignin that are intrinsic and intact in plants." They also proposed that the prior classifications of soluble versus insoluble be phased out and replaced with viscous versus fermentable with respect to each specific fiber.[31]